This invention relates to a passivating coating or collar material for metal oxide varistors (MOVs) which are primarily zinc oxide and which are useful in a valve block of a surge arrester. This invention also relates to metal oxide varistors with a passivating coating and to a method of making metal oxide varistors with a passivating coating.
Surge arresters are used to protect electrical equipment against insulation damage resulting from voltage surges which exceed the normal operating voltage of the equipment. Typically, an arrester includes a valve section and a gap section in series inside a cylindrical insulating housing. The valve section is made up of one or more metal oxide, primarily zinc oxide, varistor disks stacked face-to-face. Each of the disks of the arrester is generally provided with a passivating coating material about the periphery, or collar, to improve its stability via preventing flashover, surface conduction, corona effect, tracking, etc. under high electrical surges (e.g., switching transients, lightning, etc.) and temporary over-voltages (TOV). The passivating material is generally a ceramic material coated on the peripheral surface of the disk to provide a ceramic collar material. The ceramic based collar material must provide:
1. isolation of the MOV surfaces from the immediate environment and prevention of gas or particle diffusion;
2. high mechanical strength and/or elastic properties;
3. higher resistivity than the MOV valve block at maximum continuous operating voltage (MCOV);
4. similar dielectric constant to that of zinc oxide (i.e., .apprxeq.8.5 for MOV at extremely high electrical stress in the upturn region of the current-voltage characteristic);
5. similar thermal properties with polycrystalline zinc oxide based MOVs (i.e., thermal/heat conductivity, thermal expansion, heat diffusivity, specific heat, etc.); and
6. compact adhesion and nearly no variation in the physical gap between the collar and the MOV surfaces at any stress level.
U.S. Pat. No. 3,959,543, the disclosure of which is incorporated herein by reference, discloses a sintered zinc oxide, non-linear resistance surge arrester disk provided with a glass anti-flashover collar having a relatively low fusing temperature and a relatively low coefficient of thermal expansion. The glass comprises the following constituents in proportion to one another by weight: about 44.5% lead oxide; about 24.5% zinc oxide; about 20.5% boric oxide; about 4.5% silicon oxide; and about 6.0% cupric oxide. In making the disk, the zinc oxide composition in powdered form is pressed into a disk shape with a diameter of about 3.4 inches and a thickness of somewhat over 1 inch to form a blank. This blank is then sintered by firing in air at a temperature of about 1200.degree. C. for about five hours. Thereafter, it is cooled at about 100.degree. C. per hour, and a slurry of finely divided glass particles, having the composition as described above and mixed with ball clay to hold the glass in suspension, is applied to the outside surface about the perimeter of the disk. The disk with the applied slurry is then fired again in air at a temperature of between 640.degree. C. and 650.degree. C. for about 30 minutes to cause the glass particles in the slurry to fuse to one another and to the disk to form a collar.
U.S. Pat. No. 3,938,069, the disclosure of which is incorporated herein by reference, discloses a metal oxide varistor that includes a sintered body portion composed essentially of a metal oxide and a plurality of preselected additives. Surrounding the body portion is a passivating coating. The passivating coating preferably comprises at least some of the preselected additives. More specifically, there is described a varistor which can be fabricated from 98 mol percent zinc oxide, 0.5 mol percent bismuth oxide, 0.5 mol percent cobalt oxide, 0.5 mol percent manganese oxide, and 0.5 mol percent titanium oxide. The oxides of bismuth, cobalt, manganese and titanium are thoroughly mixed in equal molar amounts, then heated and cooled to form a crystalline solid body. The crystalline solid body is ground to form a "reaction product". Zinc oxide and the reaction product are thoroughly mixed, and the mixture is pressed and sintered to form the varistor body. The passivating coating may comprise the aforementioned reaction product alone, or in combination with other additives such as additional bismuth oxide, H.sub.3 BO.sub.3, Sb.sub.2 O.sub.3 and SiO.sub.2 . It will be appreciated that when the passivating coating comprises the reaction product by itself, it will contain, based on molar amounts, 25% Bi.sub.2 O.sub.3, 25% CO.sub.2 O.sub.3, 25% MnO.sub.2 and 25% TiO.sub.2.
The abstract of Japanese Patent No. 58-74539 discloses a vitreous composition comprising 15-35 weight percent B.sub.2 O.sub.3, 7-22 weight percent ZnO, 0-5 weight percent V.sub.2 O.sub.5, 19-39 weight percent Bi.sub.2 O.sub.3 and 19-39 weight percent Tl.sub.2 O.
The abstract of Japanese Patent No. 53-78048 discloses a glass-coated thick-film resistor comprising a thick-film resistor element coated with a low temperature crystallized glass consisting of 45-70 weight percent PbO, 5-35 weight percent Bi.sub.2 O.sub.3, 10-30 weight percent ZnO, 5-15 weight percent B.sub.2 O.sub.3 and 2-10 weight percent SnO.sub.2.